On topic of BLMs and Arrow usage for Lv70:

Disclaimer - I base this tip on the idea that the BLM in question is competent and has a proper opener for endgame.

If a BLM runs a high spellspeed build, avoid giving him an arrow at the start of a fight.

Getting the initial Foul / Polyglot right after their 8x Fire IV opener after Blizzard III is important to keep the casting flow going without relying on mp ticks, speeding this up can cause a gap in casting by lacking Polyglot for being too fast, essentially wasting the power of arrow cards. A thunder III proc can substitute it, but depends on luck or opener.

Throughout the fight it is generally a welcome gesture though (and below 70 too).